Piston compressor and method for operating a piston compressor

ABSTRACT

In order to avoid over-lubrication of the cylinders ( 2 ) in a piston compressor ( 1 ) and to reduce the amount of lubricant to a level required for operation, according to the invention, a lubricating system having a lubricating system control unit ( 14 ) is provided, in which system at least one lubricant sensor ( 15 ) is provided for detecting a lubricating film measurement variable (S) representative of a lubricating film thickness of a lubricating film ( 11 ) on the cylinder surface of the cylinder ( 2 ), the lubricating system control unit ( 14 ) being designed to operate the lubricating system at least once in a predetermined calibration operating mode during operation of the piston compressor ( 1 ) and to determine a lubricating film state value (SZ) on the basis of the lubricating film measurement variable (S) detected during execution of the calibrating operating mode and the lubricating system control unit ( 14 ) being designed, after the end of the calibration operating mode during operation of the piston compressor ( 1 ), to control the amount of lubricant to be introduced depending on the determined lubricating film state value (SZ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Austria application No. A 50536/2021, filed 28 Jun. 2021, which is incorporated herein by reference.

BACKGROUND

The invention relates to a lubricating system for a piston compressor for introducing a lubricant onto a cylinder surface of a cylinder of the piston compressor in which a piston can be moved in a reciprocating manner, a lubricating system control unit being provided for controlling an amount of lubricant to be introduced. The invention further relates to a piston compressor having a lubricating system and to a method for operating a piston compressor having at least one cylinder in which a piston is moved in a reciprocating manner, with a lubricant being supplied to a cylinder surface of the at least one cylinder by means of a lubricating system and an amount of lubricant of the supplied lubricant being controlled by a lubricating system control unit.

In piston engines, in particular in lubricated piston compressors, reliable lubrication of the cylinders is extremely important for reliable operation. There are usually one or more lubrication points in each cylinder, through which a lubricant can be introduced into the cylinder. The lubrication points are usually supplied with lubricant from a central lubricating system. Dosing the lubricant into the cylinders as precisely as possible is essential for reliable operation. Too little lubricant leads to increased wear on the moving components of the compressor, in particular on the piston rings or packing rings of sealing packings with which the piston rod is sealed. Increased wear leads to a reduced service life of these components and thus to reduced availability of the compressor. Conversely, too much lubricant tends to result in a reduced service life of components such as compressor valves due to an oil sticking effect, as well as a reduced service life of equipment connected downstream of the compressor, such as catalytic converters. In addition, high levels of lubricant naturally lead to increased operating costs due to lubricant consumption, as well as higher capital costs because additional equipment, such as special separators, is required to remove excess lubricant from the compressed process stream.

Known lubricating systems are usually based on a predetermined amount of lubricant for the relevant piston engine. These predetermined amounts of lubricant are typically provided by the compressor manufacturers depending on the compressor type, size and process parameters and are based on empirical data and/or simplified calculation models. Due to uncertainties in these calculation models and to cover all types of construction and operating conditions of the compressors, safety factors are usually provided that are chosen conservatively, so that more lubricant is usually supplied than is necessary. Such “over-lubrication” of the cylinders during operation is of course disadvantageous for the operator of a piston compressor for the reasons mentioned above and is therefore undesirable.

It is therefore an object of the invention to provide a piston compressor and a method for operating a piston compressor, with which over-lubrication of the cylinders can be avoided and the amount of lubricant can be reduced to a level required for operation.

SUMMARY OF THE INVENTION

The object is achieved using the lubricating system mentioned at the outset in that at least one lubricant sensor is provided for detecting a lubricating film measurement variable representative of a lubricating film thickness of a lubricating film on the cylinder surface of the cylinder, in that the lubricating system control unit is configured to operate the lubricating system at least once in a predetermined calibration operating mode during operation of the piston compressor and to determine a lubricating film state value on the basis of the lubricating film measurement variable detected during execution of the calibrating operating mode and, after the end of the calibration operating mode during operation of the piston compressor, to control the amount of lubricant to be introduced depending on the determined lubricating film state value. This creates a lubricating system that automatically detects the condition of the lubricating film on the cylinder liner by measuring the lubricating film thickness and evaluating the measurement result, and, based on this, automatically provides a suitable amount of lubricant. As a result, the amount of lubricant required can be significantly reduced compared to conventional lubricating systems, and therefore the cylinders are not over-lubricated.

The lubricant sensor is preferably an ultrasonic sensor, with a temporal resolution of the lubricant sensor preferably being 0.01° to 5° crank angle. This makes it possible to easily detect the lubricating film measurement variable without requiring direct access to the cylinder surface. For example, an ultrasonic sensor can easily be placed on the outside of an existing cylinder.

The lubricating system control unit is preferably configured to use a sensor value of the lubricating film measurement variable to determine the lubricating film state value, which sensor value is detected during a piston stroke of the piston at a point in time at which a piston ring of the piston is in the sensor region of the lubricant sensor, preferably a minimum value of the lubricating film measurement variable detected during the piston stroke. This allows conclusions to be drawn regarding the current lubricating film thickness in the region of a piston ring and, based on this, the required amount of lubricant can be determined.

The lubricating system is preferably configured for intermittently introducing the lubricant into the cylinder, preferably as a pump-to-point system, as a divider block system or as a common rail system, and the lubricating system control unit is configured to control the amount of lubricant by changing a frequency and/or an injection amount of each injection of the intermittent introduction of the lubricant. This means that proven lubricating systems can be used and calibrated accordingly.

The lubricating system control unit is advantageously configured to repeat the calibration operating mode in a specified cycle in order to update the lubricating film state value and to adapt the amount of lubricant to the updated lubricating film state value. As a result, changes occurring during operation which may require a greater or smaller amount of lubricant, for example wear on the piston rings, can be taken into account.

A duration of the calibration operating mode is preferably at least ten, preferably at least one hundred, particularly preferably at least one thousand, crankshaft revolutions of the piston compressor or an equivalent time. This provides sufficient time to set and evaluate different states of the lubricating film.

Preferably, at least two consecutive time ranges with different amounts of lubricant are defined in the calibration operating mode and the lubricating system control unit is configured to determine a maximum value and a minimum value in a time profile of the lubricating film measurement variable detected during the at least two time ranges and to determine therefrom the lubricating film state value in order to control the amount of lubricant. Particularly preferably, a first time range having a predetermined duration and a subsequent second time range having a predetermined duration are defined and the amount of lubricant introduced during the first time range is defined such that a completely wetted lubricating film forms on the cylinder surface and the amount of lubricant introduced during the second time range is defined such that dry running occurs on the cylinder surface. The duration of the first time range is preferably at least five crankshaft revolutions and the amount of lubricant introduced during the first time range is preferably 90-200% of an amount of lubricant predetermined by the compressor manufacturer. The duration of the second time range is preferably at least five crankshaft revolutions and the amount of lubricant introduced during the second time range is preferably 0% of the amount of lubricant predetermined by the compressor manufacturer. This simulates various lubricating film conditions and is used to determine a representative, generally applicable lubricating film state value.

It is advantageous if the lubricating system control unit is configured to determine a difference value between the determined maximum value and the determined minimum value, to determine a lubricating film limit value from the maximum value and the determined difference value, and to use the lubricating film limit value as a lubricating film state value. The lubricating system control unit is preferably also configured, during operation of the piston compressor after the end of the calibration operating mode, to control the lubricating system for introducing the lubricant when the lubricating film measurement variable lies in a lubricating system activation range between the minimum value and the lubricating film limit value. On the one hand, this creates a differential evaluation method that is substantially independent of the detected absolute values of the detected lubricating film measurement variable and a simple indicator for activating the introduction of lubricant is provided.

A lubricant amount detection unit is preferably also provided in the lubricating system for detecting an amount of lubricant supplied to the lubrication point, and the lubricating system control unit is designed to compare the lubricating film measurement variable obtained from the lubricating film sensor and the amount of lubricant obtained from the lubricant amount detection unit in order to check the two values for consistency and/or to detect a leak in the lubricating system. This allows the function of the lubricant sensor to be checked and leaks in the lubricant lines to be detected. Based on this, certain actions, such as switching off the compressor or changing the lubrication to conventional over-lubrication, can be carried out, as a result of which operational reliability can be increased.

The invention is also achieved using a method in that at least one lubricant sensor is used to detect a lubricating film measurement variable representative of a lubricating film thickness of a lubricating film on the cylinder surface of the cylinder, in that the lubricating system is operated at least once by the lubricating system control unit in a predetermined calibration operating mode during operation of the piston compressor, a lubricating film state value being determined on the basis of the lubricating film measurement variable detected during execution of the calibrating operating mode, and in that the lubricating system control unit, after the end of the calibration operating mode during operation of the piston compressor, controls the amount of lubricant to be introduced depending on the determined lubricating film state value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in greater detail below with reference to FIGS. 1 to 4 , which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

In the drawings:

FIG. 1 is a section through a cylinder of a piston compressor,

FIG. 2 is a diagram with a detected lubricating film measurement variable over the crank angle,

FIG. 3 a is a diagram with a time profile of the detected lubricating film measurement variable over the compressor revolutions of the piston compressor,

FIG. 3 b is a diagram with a percentage amount of lubricant over the compressor revolutions of the piston compressor,

FIG. 4 is a diagram with a detected lubricating film measurement variable plotted over the crank angle of the piston compressor.

FIG. 1 shows a simplified sectional view through a cylinder 2 of a piston compressor 1. A piston 3 is arranged in a known manner in the cylinder 2 and can be moved in a reciprocating manner in the cylinder between a top dead center OT and a bottom dead center UT. The piston 3 can be driven in a known manner by a crankshaft (not shown) via a connecting rod (not shown), a crosshead (not shown) and a piston rod 4. Of course, other designs of the piston compressor 1 would also be possible, for example direct driving of the piston 3 by a connecting rod without a crosshead and piston rod 4. In the example shown, a cylinder sleeve 5, a so-called liner, is arranged in the cylinder 2, on the inner circumferential surface of which a cylinder surface for the piston 3 is formed.

In principle, however, an embodiment without a cylinder liner 5 is also possible, in which the cylinder surface is provided directly on the inner circumferential surface of the cylinder 2. A piston compressor 1 can, of course, have a plurality of cylinders 2, in each of which a piston 3 can be moved back and forth, it being possible for the plurality of pistons 3 to be driven by a common crankshaft. One or more piston rings 6 can be provided on the piston 3 and are arranged in suitable circumferential grooves on the circumferential surface of the piston 3. Within the context of the invention, a piston ring 6 is generally to be understood to mean piston rings having different functions. For example, a piston ring 6 can be designed as a sealing ring, as a support ring or as a scraper ring. A sealing ring is designed, for example, to seal against a differential pressure, while a support ring usually has no sealing effect and is designed to support the load of the piston 3 on the cylinder liner. A scraper ring, in turn, is designed to scrape the lubricating film from the cylinder surface.

Of course, not all designs have to be provided on a piston 3, but only one piston ring 6 could be provided, for example in the form of a sealing ring. In the example shown, three piston rings 6 a, 6 b, 6 c are provided, the first and third piston rings 6 a, 6 c being designed as sealing rings and the second piston ring 6 b being designed as a support ring. However, more or fewer piston rings 6 could also be provided, for example an oil scraper ring. It can be seen here that the support ring 6 b has a greater width (in the axial direction) than the sealing rings 6 a, 6 c, which is usually the case. In addition, the groove in the piston 3 in which the support ring 6 b is arranged is designed in such a way that the support ring 6 b rests directly on the bottom of the groove. In contrast to the sealing rings 6 a, 6 c, the support ring 6 b is thus substantially immovable in the radial direction in order to be able to better support the load of the piston 3. The sealing rings 6 a, 6 c, on the other hand, can be moved radially in the respective grooves in order to be able to create a better seal. In general, a piston ring 6 can, of course, also have a certain overhang over the piston 3 in the radial direction, which is not shown in FIG. 1 for the sake of simplicity.

In the cylinder 2, a compression chamber 7 is formed in a known manner which is delimited by the piston 3 and on which a suction valve 8 and a pressure valve 9 are arranged. During an expansion stroke, the piston 3 is moved from top dead center OT to bottom dead center UT, and during a compression stroke, the piston 3 is moved from bottom dead center UT to top dead center OT. During the expansion stroke, a gaseous medium to be compressed, for example air or a process gas, can be sucked into the compression chamber 7 via the suction valve 8. During the compression stroke, the medium in the compression chamber 7 is compressed and discharged from the compression chamber 7 via the pressure valve 9. The suction valve 8 and the pressure valve 9 are indicated in FIG. 1 merely as schematic circuit symbols and can be designed in different ways.

Several suction valves 8 and pressure valves 9 can also be provided on the cylinder. The suction valve 8 and the pressure valve 9 do not necessarily have to be arranged on the end face of the cylinder 2, but could also be provided on the circumferential surface of the cylinder 2 in the compression chamber 7, for example. For example, the suction valve 8 and the pressure valve 9 can be designed as known automatic ring valves, it also being possible for an unloader to be provided to keep the valves open. The unloader can be controlled by a suitable compressor control unit 10 to regulate the capacity of the compressor 1.

The piston compressor 1 also has a lubricating system for lubricating the at least one cylinder 2. For this purpose, the lubricating system has at least one lubrication point 12 for each cylinder 2 for introducing a lubricant into the cylinder 2. The lubricant forms a lubricating film 11 in the cylinder 2 in order to minimize the friction between the components that move relative to one another, in particular between the piston 3 or piston rings 6 and the cylinder surface. In order to allow the lubricant to be distributed as evenly as possible in the cylinder 2, several lubrication points 12 can of course also be provided, which can, for example, be arranged spaced apart in the axial direction and/or in the circumferential direction on the cylinder 2, as indicated in FIG. 1 by the lubrication points 12a, 12b. Depending on the structural design of a piston ring 6, it can be advantageous if the lubricant is introduced via the lubrication point 12 in the compression stroke and/or in the expansion stroke, for example before the relevant piston ring 6 reaches the lubrication point 12, while the piston ring 6 is in the region of the lubrication point 12 or possibly also after the relevant piston ring 6 has passed the lubrication point 12. The point in time at which it is introduced substantially depends on the lubricating system used.

Various lubricating systems are known in the prior art, including “divider block” systems, “pump-to-point” systems and “common rail” systems. In a “divider block” system, a central delivery unit for delivering the lubricant and a “divider block” for distributing the lubricant to the lubrication points 12 of the cylinders 2 are provided. The amount of lubricant conveyed by the central conveying unit is fed to what is known as a “divider block,” divided up therein and conveyed to the individual lubrication points 12. The lubricant is generally introduced into the cylinder 2 intermittently via individual injections. The amount of lubricant of the lubricant to be introduced is controlled by appropriate control of the central delivery unit. If the central delivery unit is designed as a piston pump, for example, the delivery amount can be controlled via the speed and/or optionally via the piston stroke. A change in the amount of lubricant during operation of the compressor 1 can take place, for example, by changing the frequency of the intermittent individual injections of the lubricant, for example by changing the pump speed. Due to the central delivery unit, however, the amount of lubricant can be changed only in a constant ratio for all available lubrication points 12. Different amounts of lubricant for different lubrication points 12 or different cylinders 2 are not usually possible.

In a “pump-to-point” system, each lubrication point 12 or each cylinder 2 is assigned its own delivery unit, e.g. a piston pump. Depending on the structural design of the delivery unit, for example the stroke or displacement of the piston pump, a corresponding amount of lubricant is delivered to the associated lubrication point 12, again generally intermittently over individual injections. Generally, the delivery units, in particular the piston pumps, are driven via a common camshaft. A change in the amount of lubricant during operation of the compressor 1 can take place, for example, by changing the frequency of the individual injections of the lubricant by adjusting the speed of the camshaft. A change in the amount of lubricant usually only occurs in a constant ratio for all lubrication points 12. With some “pump-to-point” systems, however, the delivery rates can also be changed separately from one another, for example by an adjustable stroke of the piston pumps. As a result, the amount of lubricant at each lubrication point 12 or for each cylinder 2 can be adjusted individually. Suitable adjusting devices can be provided on the delivery units, for example, in order to change the amount of lubricant. The adjusting device can be designed for manual adjustment of the stroke of a piston pump, for example, or a suitable actuator can be provided for adjusting the stroke, so that the amount of lubricant introduced can be changed per injection.

There are also “common rail” systems, in which the lubricant is pumped into a pressure accumulator by a high-pressure pump and can be supplied individually from the pressure accumulator to each lubrication point 12 or each cylinder 2 via pressure lines by means of electrically controllable injectors. In comparison to the previously mentioned systems, such systems offer the greatest degree of freedom in controlling the introduction of the lubricant. In particular, the amount of lubricant can be changed not only by changing the frequency of the intermittent injection, but the amount of lubricant introduced per injection can also be controlled in a very precise and variable manner. In addition, the time of introduction at each lubrication point 12 or at each cylinder 2 can be adjusted individually and substantially independently of the operation of the high-pressure pump (as long as there is sufficient pressure in the pressure accumulator). This makes it possible, for example, for a piston compressor 1 of which the pistons 3 have a plurality of piston rings 6 to carry out multiple injections within one piston stroke, so that each piston ring 6 can be supplied with a specific amount of lubricant.

In all lubricating systems, the control of the lubricant injection, for example the time of an injection and/or the amount of lubricant per injection and/or the frequency of the injections, is usually carried out via a suitable lubricating system control unit 14. The lubricating system control unit 14 can be designed as separate hardware and/or software or can also be integrated in a higher-level control unit such as the compressor control unit 10. In FIG. 1 , a “common rail” system is shown merely by way of example, with one electrically controllable injector 13 being provided for each lubrication point 12, which injector can be designed, for example, as an electromagnetic injector or as a piezo injector. The injectors are connected to a central lubricating system control unit 14 via a suitable communication connection, such as electrical wiring. As mentioned above, a pressure accumulator and a high-pressure pump are also provided, but these are not shown in FIG. 1 for the sake of simplicity. The high-pressure pump is preferably also controlled by the lubricating system control unit 14. The lubricating system control unit 14 can, for example, communicate with a compressor control unit 10 in order to obtain operating parameters BP of the piston compressor 1 that are relevant for the control of the lubricating system. Such operating parameters BP can, for example, contain current data regarding the operating state of the compressor 1, for example a load signal L, a speed signal N, a crank angle signal ° KW, lubricant temperature T, etc. The lubricating system control unit 14 can take the operating parameters BP into account when controlling the lubricating system.

As mentioned at the outset, the amount of lubricant used to be controlled on the basis of manufacturer specifications, which often led to over-lubrication, i.e. to the introduction of a greater amount of lubricant than necessary. In order to avoid this, an automatic calibration of the lubricating system is provided according to the present invention, as a result of which the amount of lubricant can be adapted to the actual requirement. For this purpose, at least one lubricant sensor 15 is provided in the lubricating system for detecting a lubricating film measurement variable S representative of a lubricating film thickness of the lubricating film 11 on the cylinder surface of the cylinder 2. The lubricating film measurement variable S is essentially a measure of the lubricating film thickness of the lubricating film 11 on the cylinder surface in a sensor region of the lubricant sensor 15. The lubricant sensor 15 is connected to the lubricant control unit 14 via a suitable communication connection in order to transmit the measured lubricating film measurement variable S to the lubricant control unit 14, for example via suitable electrical measuring lines. An acoustic sensor, in particular an ultrasonic sensor, is preferably provided as the lubricant sensor 15. The lubricant sensor 15 can be arranged at a suitable location on the outside of the cylinder 2, for example.

The lubricant sensor 15 is preferably arranged in the axial direction such that each piston ring 6 of the piston 3 is located once in the sensor region of the lubricant sensor 15 for each piston stroke, so that a lubricating film thickness in the region of each piston ring 6 can be detected. Analogously to the plurality of lubrication points 12, a plurality of lubricant sensors 15 can also be arranged on the cylinder 2 at a distance from one another in the circumferential direction and/or in the axial direction. This can be particularly advantageous in the case of large compressors in order to cover a sufficiently large area of the cylinder surface of the cylinder 2. The temporal resolution of the lubricant sensor 15, in particular the ultrasonic sensor, for detecting the lubricating film measurement variable S may be, for example, 0.01° to 5° crank angle.

According to the invention, the lubricating system control unit 14 is designed to operate the lubricating system at least once in a predetermined calibration operating mode during operation of the piston compressor 1 and to determine a lubricating film state value SZ on the basis of the lubricating film measurement variable S detected during execution of the calibration operating mode. After the end of the calibration operating mode, i.e. during normal operation of the lubricating system during operation of the piston compressor 1, the lubricating system control unit 14 controls the amount of lubricant to be introduced depending on the determined lubricating film state value SZ.

As mentioned, the lubricating system is preferably designed for the intermittent introduction of the lubricant, preferably as a pump-to-point system, as a divider block system or as a common rail system. The lubricating system control unit 14 can adjust the amount of lubricant depending on the determined lubricating film state value SZ, for example by changing the frequency of the intermittent introduction of the lubricant, and thereby reduce the amount of lubricant compared to the manufacturer's specification. If the lubricating system is designed appropriately, e.g. in a common rail system, the (total) amount of lubricant introduced into the cylinder 2 can also be changed by changing the amount of lubricant per injection of the injector 13 in addition or as an alternative to changing the frequency. In the case of pump-to-point or divider block systems, the frequency can be changed, as mentioned, for example by increasing the speed of the feed pump(s) of the relevant lubricating system.

A sensor value Pi of the measurement variable S of the lubricating film is preferably used to determine the lubricating film state value SZ, which sensor value is detected during a piston stroke of the piston 3 at a point in time at which a piston ring 6 of the piston 3 is in the sensor region of the lubricant sensor 15. The minimum value of the lubricating film measurement variable S is preferably used in each case, as shown in FIG. 2 . FIG. 2 shows an example of a profile of the detected lubricating film measurement variable S of a lubricant sensor 15 over the crank angle ° KW between top dead center OT and bottom dead center UT of the piston 3 during normal operation of the piston compressor 1 (and the lubricating system). The piston 3 corresponds to the embodiment according to FIG. 1 and accordingly has three piston rings 6 a, 6 b, 6 c. Of course, more or fewer piston rings 6 could also be provided. The curve shown corresponds to the measurement signal of the lubricant sensor 15 over the crank angle. As can be seen in FIG. 2 , depending on the number i of piston rings 6 i, there is a characteristic time profile of the lubricating film measurement variable S, which can be used as a measure of the lubricating film thickness of the lubricating film 11 on the cylinder surface of the cylinder 2.

The curve has a local minimum for each piston ring 6 i which is proportional to the lubricating film thickness of the lubricating film 11 on the cylinder surface when the relevant piston ring 6 i is in the sensor region of the lubricant sensor 15. In the example shown, these are the minimum values Pa, Pb, Pc, which are generally referred to below as PEAK values Pi within the context of the invention. The assignment of the PEAK values Pa, Pb, Pc to the relevant piston ring 6 a, 6 b, 6 c results from their arrangement on the piston 3. According to the invention, these PEAK values Pa, Pb, Pc (generally Pi) can now be used in a calibration operating mode of the lubricating system to determine the lubricating film state value SZ, as explained below with reference to FIG. 3 a +3 b.

In principle, however, more than one lubricant sensor 15 could of course also be provided in the lubricating system in order to allow redundant detection of the lubricating film measurement variable S. The lubricant sensors 15 can, for example, be arranged at a certain angular distance in the circumferential direction on the cylinder 2 and/or can be arranged on the cylinder 2 at a distance in the axial direction. Of course, each lubricant sensor 15 is preferably arranged in such a way that all available piston rings 6 i of the piston 3 are in the sensor region of the relevant lubricant sensor 15 once during a piston stroke. Two lubricant sensors 15 with different positions in the circumferential direction and the same axial positions on the cylinder 2 would result, for example, in qualitatively identical curves of the lubricating film measurement variable S, which, however, can differ quantitatively due to the locally different lubricating film thickness of the lubricating film 11. However, the PEAK values Pi would be at the same crank angle position. On the other hand, two lubricant sensors 15 with different positions in the axial direction and the same positions in the circumferential direction would produce, for example, qualitatively and quantitatively different profiles of the lubricating film measurement variable S. In this case, the PEAK values Pi would lie at different crank angle positions. However, the advantageous differential evaluation method of the PEAK value curves, explained in more detail below, would again compensate for the different absolute values of the PEAK values Pi.

For example, the PEAK values Pi may be sampled once every crankshaft revolution or once every piston stroke and stored in the lubricating system controller 14. In principle, however, it can also be sufficient if the PEAK values Pi are not detected uninterruptedly, i.e. not detected for each crankshaft revolution or each piston stroke and stored for the evaluation of the time profile, but rather the PEAK values Pi are detected intermittently, for example, with a specified interruption duration of a few crankshaft revolutions or, for example, a time of 1 to 60 seconds.

An example of a calibration operating mode of the lubricating system is described below with reference to FIGS. 3 a and 3 b . FIG. 3 b shows a time profile of the amount of lubricant introduced into the cylinder 2 during the calibration operating mode over the crankshaft revolutions of the compressor 1. In FIG. 3 a , using the example of the first piston ring 6 a, a time profile of the stored PEAK values Pa over the crankshaft revolutions of the compressor 1 is shown, which results from the amount of lubricant introduced according to FIG. 3 b . A duration of the calibration operating mode may be, for example, at least ten, preferably at least one hundred, particularly preferably at least one thousand, crankshaft revolutions of the piston compressor 1. Since the crankshaft revolutions are proportional to the time, the time can in principle also be plotted on the abscissa.

According to the invention, the calibration operating mode is executed at least once during operation of the piston compressor 1, for example after a running-in phase during initial start-up or after a service activity on the compressor 1. The running-in phase typically corresponds to an operating time of 12 to 36 hours. Of course, the calibration operating mode can also be repeated several times in a specified cycle in order to determine an updated lubricating film state value SZ and to adapt the amount of lubricant to the updated lubricating film limit value SZ. As a result, different states of wear that occur during operation, which are usually associated with a change in the need for lubricant, can be taken into account.

In the calibration operating mode, preferably at least two consecutive time ranges Zi with different amounts Mi of lubricant are defined and the lubricating system control unit 14 determines a maximum value Pi_max and a minimum value Pi_min in a time profile of the lubricating film measurement variable S detected during the at least two time ranges, in particular the PEAK values Pi, and uses this to determine the lubricating film state value SZ (FIG. 4 ), which is used to control the amount of lubricant. The maximum value Pi_max and the minimum value Pi_min and the associated detection times are preferably determined and (at least temporarily) stored. The storage can generally take place, for example, in a suitable memory unit which can be integrated, for example, in the lubricating system control unit 14 or in a higher-level compressor control unit 10. The time ranges Zi and the amounts of lubricant Mi introduced in the time ranges Zi are defined in such a way that different friction and associated states of wear occur on the cylinder surface, from a fully wetted lubricating film to a partially wetted lubricating film to dry running. The amount M of lubricant is indicated on the ordinate in the diagram in FIG. 3 b as a percentage of an amount of lubricant predetermined by the compressor manufacturer.

In the calibration operating mode shown, a first time range Z1 having a predetermined duration and a subsequent second time range Z2 having a predetermined duration are defined only by way of example. The amount M1 of lubricant introduced during the first time range Z1 is preferably defined such that a completely wetted lubricating film is formed on the cylinder surface. The amount M2 of lubricant introduced during the second time range Z2, on the other hand, is preferably defined such that dry running occurs on the cylinder surface. For example, the duration of the first time range Z1 can be at least five crankshaft revolutions and the amount M1 of lubricant introduced during the first time range Z1 can be 90-200% of an amount of lubricant predetermined by the compressor manufacturer. As a result, the maximum value Pa_max is generally in the first time range Z1 and the minimum value Pa_min is generally in the second time range Z2. At the transition between the first time range Z1 and the second time range Z2, there is a characteristic drop in the profile of the PEAK values Pa, as can be seen in FIG. 3 a .

The duration of the second time range Z2 is preferably also at least five crankshaft revolutions, but can of course also be significantly longer, for example ten, one hundred or one thousand crankshaft revolutions. The amount M2 of lubricant introduced during the second time range Z2 is preferably 0% of the amount of lubricant predetermined by the compressor manufacturer, and therefore no lubricant is introduced. At the end of the second time range Z2, the amount M of lubricant can be increased again, for example initially to the amount M3 of lubricant predetermined by the compressor manufacturer, as indicated by the third time range Z3. In principle, the calibrating operating mode is completed after the second time range Z2 and from this point in time the lubricating system control unit 14 can use the lubricating film state value SZ (FIG. 4 ) determined during the calibration operating mode to control the amount of lubricant during operation of the piston compressor 1.

Advantageously, a differential value APa is first determined between the determined maximum value Pa_max and the determined minimum value Pa_min, as shown in FIG. 3 a . A lubricating film limit value Pa_grenz can now be determined on the basis of the maximum value Pa_max and the determined differential value ΔPa, which limit value can advantageously be used as the lubricating film state value SZ for controlling the amount of lubricant, as described below with reference to FIG. 4 . The limit value Pa_grenz can be determined, for example, according to the following relationship: Pa_grenz=Pa_max−k*ΔPa , with a percentage factor K which can range from 10% to 100%. If a plurality of lubricant sensors 15 are provided on a cylinder 2, the evaluation and determination of the lubricating film state value SZ, in particular the limit value Pi_grenz, is of course preferably carried out for the lubricating film measurement variable S of each lubricant sensor 15. However, mean values of the maximum values Pi_max and the minimum values Pi_min of a plurality of lubricant sensors 15 could also be used, for example, in order to calculate a mean limit value Pi_grenz therefrom. Likewise, a mean value could also be formed from a plurality of limit values Pi_grenz, and the amount of lubricant could be controlled on this basis.

FIG. 4 , analogously to FIG. 2 , shows a profile of the detected lubricating film measurement variable S over the piston stroke between a top dead center OT and a bottom dead center UT during normal operation of the piston compressor 1 and in particular during normal operation of the lubricating system (after the end of or outside the calibration operating mode). In addition, the maximum value Pa_max and minimum value Pa min determined beforehand in the calibration operating mode as well as the lubricating film limit value Pa_grenz are shown in FIG. 4 . The hatched area between the minimum value Pa_min and the lubricating film limit value Pa_grenz symbolizes a lubricating system activation range 17. If the lubricating system control unit 14 recognizes that the PEAK value Pa of the detected lubricating film measurement variable S is in the lubricating system activation range 17, i.e. the PEAK value Pa reaches or falls below the lubricating film limit value Pa_grenz, then the lubricating system control unit 14 controls the lubricating system for introduction of the lubricant.

This means that no lubricant is introduced into the cylinder 2 via the lubrication point(s) 12 as long as the PEAK value Pa of the detected lubricating film measurement variable S is outside the lubricating system activation range 17 (i.e. above the limit value Pa_grenz). Lubricant is supplied back to the cylinder 2 only when the PEAK value Pa of the detected lubricating film measurement variable S is sufficiently low and within the lubricating system activation range 17 (which is an indicator of insufficient lubricating film thickness of the lubricating film 11). After an injection process of the lubricant has been carried out, the characteristic profile of the lubricating film measurement variable S (FIG. 4 ) will be flatter again, and therefore the PEAK value Pa is above the limit value. Depending on the lubricant used, the amount of lubricant introduced during the injection process, as well as the operating state and state of wear of the piston compressor 1, etc., the characteristic profile of the lubricating film measurement variable S will change again over the crankshaft revolutions, so that the PEAK value Pa slowly returns to the lubricating system activation range 17.

As soon as the PEAK value Pa falls back into the lubricating system activation range 17, another injection process is triggered etc. by the lubricating system control unit 14. The (total) amount of lubricant is thus substantially controlled by adjusting the frequency of the individual injections. In common rail systems, in addition to changing the frequency, the amount of lubricant introduced per injection could also be varied. In the case of pump-to-point and divider block systems, on the other hand, the amount of lubricant introduced per injection substantially depends on the design of the delivery unit(s) (e.g. the displacement of a piston pump) and is usually unchangeable. An adjustment of the (total) amount of lubricant can thus generally only be controlled by adjusting the frequency of the individual injections, for example by changing the speed of the piston pump(s).

The description based on the first piston ring 6 a is of course only to be understood by way of example and the calibration operating mode could of course also be carried out separately for a plurality of piston rings 6 i. For example, in the calibration operating mode, the profile of the lubricating film measurement variable S of the lubricant sensor 15 can be evaluated for each piston ring 6 i in order to determine a lubricating film state value SZi for each piston ring 6 i for controlling the amount of lubricant. For example, a limit value Pi_grenz can be determined for each piston ring 6 i, so that an associated lubricating system activation range 17 i is determined for each piston ring 6 i. The lubricating system control unit 14 can then, during normal operation of the piston compressor 1, control the lubricating system to introduce lubricant as soon as the relevant PEAK value Pi is in the respectively assigned lubricating system activation range 17. The limit value Pi_grenz can of course also differ.

If, for example, a piston ring 6 i has a higher lubricant requirement than another piston ring 6 i, it can also be sufficient if the calibration according to the invention is only carried out for the piston ring 6 i with the higher lubricant requirement. The limit value Pi_grenz would thus only be determined for one piston ring 6 i and the lubricating system control unit 14 would activate the lubricating system accordingly as soon as the assigned PEAK value Pi is in the determined lubricating system activation range 17 i. If the lubricating system is suitable for this (for example a common rail system), the introduction of the lubricant is preferably timed in such a way that the lubricant is supplied precisely to the piston ring 6 i with the greatest need for lubricant. Depending on the structural design of the piston ring 6 i, the lubricant is preferably introduced during the piston stroke in front of the piston ring 6 i or directly onto the piston ring 6 i. The piston ring or rings 6 i with the lower lubricant requirement consequently automatically receive a sufficiently large amount of lubricant. If the piston compressor 1 has a plurality of cylinders 2, then the detection and evaluation of the amount of lubricant according to the invention preferably takes place individually for each cylinder 2 by arranging at least one lubricant sensor 15 on each cylinder 2.

Furthermore, it can be advantageous if a lubricant amount detection unit is provided in the lubricating system for detecting an amount of lubricant supplied to the lubrication point 12. The lubricant amount detection unit can be a suitable flow sensor 16, for example, which is integrated in a feed line to a lubrication point 12, as indicated in FIG. 1 . The flow sensor 16 is preferably connected to the lubricating system control unit 14 in order to transmit a measurement signal which is proportional to the amount of lubricant. However, a calculation model could also be provided as the lubricant amount detection unit, for example, which can be implemented in the lubricating system control unit 14, for example, and which calculates the amount of lubricant supplied to the lubrication point 12 using available parameters of the lubricating system, for example based on the displacement of a piston pump and the pump speed, etc.

The lubricating system control unit 14 can therefore compare the lubricating film measurement variable S obtained from the lubricating film sensor 15 and the amount of lubricant obtained from the lubricant amount detection unit in order to check the two values for consistency. As a result, a malfunction of the lubricant sensor 15 can be inferred, for example, if a certain discrepancy between the two values is determined. Likewise, the comparison can be used to detect a leak in the lubricating system. A leak may be present, for example, when the flow sensor 16 outputs a certain expected value and the lubricating film measurement variable S detected by the lubricant sensor 15 assumes no value or a very low value. This may mean, for example, that there is a leak between the flow sensor 16 and the lubrication point 12. If, for example, an impermissibly high deviation between amount measurement and lubricating film thickness measurement (or comparable variables derived therefrom) is determined, this can be used, for example, as an indicator of a critical operating state of the piston compressor and certain actions can be initiated, such as an alarm signal or switching off the compressor 1. 

1. Lubricating system for a piston compressor for introducing a lubricant onto a cylinder surface of a cylinder of the piston compressor in which a piston can be moved in a reciprocating manner, the lubricating system comprising: a lubricating system control unit configured to control an amount of lubricant to be introduced, at least one lubricant sensor configured to detect a lubricating film measurement variable representative of a lubricating film thickness of a lubricating film on the cylinder surface of the cylinder, wherein the lubricating system control unit is further configured to operate the lubricating system at least once in a predetermined calibration operating mode during operation of the piston compressor, determine a lubricating film state value on the basis of the lubricating film measurement variable detected during execution of the calibrating operating mode, and after the end of the calibration operating mode, during operation of the piston compressor, control the amount of lubricant to be introduced depending on the determined lubricating film state value.
 2. The 1 lubricating system according to claim 1, characterized in that the lubricant sensor is an ultrasonic sensor.
 3. The lubricating system according to claim 1, characterized in that the lubricating system control unit is further configured to use a sensor value of the lubricating film measurement variable to determine the lubricating film state value, the sensor value is detected during a piston stroke of the piston at a point in time at which a piston ring of the piston is in the sensor region of the lubricant sensor.
 4. The lubricating system according to claim 1, characterized in that the lubricating system is configured for an intermittent introduction of the lubricant into the cylinder, and the lubricating system control unit is further configured to control the amount of lubricant by changing a frequency and/or an injection amount of each injection of the intermittent introduction of the lubricant.
 5. The lubricating system according to claim 1, wherein the lubricating system control unit is further configured to repeat the calibration operating mode in a specified cycle in order to update the lubricating film state value and to adapt the amount of lubricant to the updated lubricating film state value.
 6. The lubricating system according to claim 1, characterized in that a duration of the calibration operating mode is at least ten, crankshaft revolutions of the piston compressor or an equivalent time.
 7. The lubricating system according to claim 1, wherein the lubrication system control unit is further configured and arranged to operate the lubricating system in at least two consecutive time ranges with different amounts of lubricant in the calibration operating mode, determine a maximum value and a minimum value in a time profile of the lubricating film measurement variable detected during the at least two time ranges and to determine therefrom the lubricating film state value for controlling the amount of lubricant.
 8. The lubricating system according to claim 7, wherein the lubrication system control unit is further configured and arranged to operate the lubricating system in a first time range having a predetermined duration and a subsequent second time range having a predetermined duration, wherein the amount of lubricant introduced during the first time range is defined such that a completely wetted lubricating film sets on the cylinder surface, and the amount of lubricant introduced during the second time range is set such that dry running occurs on the cylinder surface.
 9. The lubricating system according to claim 7, characterized in that the lubricating system control unit is further configured to determine a difference value between the determined maximum value and the determined minimum value, determine a lubricating film limit value from the maximum value and the determined difference value, use the lubricating film limit value as a lubricating film state value and during operation of the piston compressor after the end of the calibration operating mode, introduce the lubricant when the lubricating film measurement variable lies in a lubricating system activation range between the minimum value and the lubricating film limit value.
 10. The lubricating system according to claim 1, further including a lubricant amount detection unit configured to detect an amount of lubricant supplied to a lubrication point and in that the lubricating system control unit is configured to compare the lubricating film measurement variable obtained from the lubricating film sensor and the amount of lubricant obtained from the lubricant amount detection unit in order to check the two values for consistency and/or to detect a leak in the lubricating system.
 11. A piston compressor comprising: a number of cylinders, a number of pistons, each piston arranged in one of the cylinders and configured to move in a reciprocating manner within the respective cylinder, at least one lubrication point provided on each of the number of cylinders, and configured to introduce the lubricant onto a cylinder running surface of the cylinder, and a lubricating system in accordance with claim 1 configured to supply the number of cylinders with the lubricant.
 12. The piston compressor according to claim 11, characterized in that a plurality of lubrication points and/or a plurality of lubricant sensors are provided on at least one cylinder.
 13. Method for operating a piston compressor having at least one cylinder in which a piston is moved in a reciprocating manner, wherein a lubricant is supplied to a cylinder surface of the at least one cylinder by means of a lubricating system and an amount of lubricant of the supplied lubricant being controlled by a lubricating system control unit, the method including the following steps: using at least one lubricant sensor to detect a lubricating film measurement variable representative of a lubricating film thickness of a lubricating film on the cylinder surface of the cylinder, operating the lubricating system at least once by the lubricating system control unit in a predetermined calibration operating mode during operation of the piston compressor, determining a lubricating film state value on the basis of the lubricating film measurement variable detected during execution of the calibrating operating mode, and in that the lubricating system control unit, after the end of the calibration operating mode during operation of the piston compressor, controls the amount of lubricant to be introduced depending on the determined lubricating film state value.
 14. The method according to claim 13, characterized in that an ultrasonic sensor is used as the lubricant sensor.
 15. The method according to claim 13, characterized in that a sensor value of the lubricating film measurement variable is used to determine the lubricating film state value, the sensor value is detected during a piston stroke of the piston at a point in time at which a piston ring of the piston is in the region of the lubricant sensor.
 16. The method according to claim 13, characterized in that the lubricant is intermittently introduced into the cylinder and in that the lubricating system control unit controls the amount of lubricant by changing a frequency and/or an injection amount of each injection of the intermittent introduction of the lubricant.
 17. The method according to claim 13, characterized in that the calibration operating mode is repeated in a specified cycle in order to update the lubricating film state value and to adapt the amount of lubricant to the updated lubricating film state value and/or in that the calibration operating mode is executed over at least ten, crankshaft revolutions of the piston compressor or an equivalent time.
 18. The method according to claim 13, characterized in that in the calibration operating mode, different amounts of lubricant are introduced into the cylinder in at least two consecutive time ranges and in that the lubricating system control unit determines a maximum value and a minimum value in a time profile of the lubricating film measurement variable detected during the at least two time ranges and determines therefrom the lubricating film state value for controlling the amount of lubricant.
 19. The method according to claim 18, characterized in that a first time range having a predetermined duration and a subsequent second time range having a predetermined duration are defined and in that the amount of lubricant introduced during the first time range is defined such that a completely wetted lubricating film forms on the cylinder surface and the amount of lubricant introduced during the second time range is defined such that dry running occurs on the cylinder surface, the duration of the first time range and the amount of lubricant introduced during the first time range being 90-200% of an amount of lubricant predetermined by the compressor manufacturer and the duration of the second time range and the amount of lubricant introduced during the second time range being 0% of the amount of lubricant predetermined by the compressor manufacturer.
 20. The method according to claim 18, characterized in that a difference value between the determined maximum value and the determined minimum value is determined, a lubricating film limit value is determined from the maximum value and the determined difference value, and the lubricating film limit value is used as a lubricating film state value and in that the lubricating system control unit, after the end of the calibration operating mode, controls the lubricating system for introducing the lubricant when the lubricating film measurement variable lies in a lubricating system activation range between the minimum value and the lubricating film limit value.
 21. The lubricating system according to claim 8, wherein the duration of the first time range and the amount of lubricant introduced during the first time range being 90-200% of an amount of lubricant predetermined by the compressor manufacturer and the duration of the second time range and the amount of lubricant introduced during the second time range being 0% of the amount of lubricant predetermined by the compressor manufacturer.
 22. The piston compressor according to claim 12, wherein the plurality of lubrication points and/or lubricant sensors being provided in the circumferential direction of the at least one cylinder and/or a plurality of lubrication points and/or lubricant sensors being provided in the axial direction of the at least one cylinder.
 23. The method according to claim 15, wherein a minimum value of the lubricating film measurement variable (S) detected during the piston stroke.
 24. The method according to claim 19, the duration of the first time range being at least five crankshaft revolutions and the amount of lubricant introduced during the first time range being 90-200% of an amount of lubricant predetermined by the compressor manufacturer and the duration of the second time range being at least five crankshaft revolutions and the amount of lubricant introduced during the second time range being 0% of the amount of lubricant predetermined by the compressor manufacturer. 